The specific aims of this research are to investigate those areas of intrinsic factor (IF) mediated absorption of vitamin B12 (cobalamin) that are as yet not clearly understood. To do this, I shall use radiolabeled IF that can now be prepared in our laboratory. The luminal degradation of IF will be studied in detail, since several important physiological processes depend on the availability of non-degraded IF in the gut lumen. Labeled IF-Cbl will also be used in subcellular fractionation studies to determine the fate of IF and Bcl during the transcellular transport of Cbl. In addition, a morphologic study will be carried out, based on the biotin-avidin system, to examine whether intact IF-Cbl enters the ileal cell during Cbl absorption. Recent evidence has been presented to show that in its passage through the enterocyte, Cbl binds to a transcobalamin II (TCII)-like protein. Studies will be carriedd out to determine whether this protein is identical to plasma TCII and whether its de novo synthesis can be accomplished by isolated cells and mucosa. A knowledge of these cellular events of Cbl absorption should enable a clearer understanding of the malabsorption of this vitamin that occurs during the prolonged ingestion of drugs such as PAS and the biguanides as well as in certain congenital disorders like the Imerslund-Grasbeck syndrome and congenital transcobalamin II deficiency.